I bought the flex for my first polisher, granted I tryed both out at Adams before I bought (benefit of living nearby Adams:rockon:) and had Mook showing me how to use it, but the videos are really good at explaining what you need to know, and the forums have the rest.
In my opinion the PC is actually harder to control than the flex, and it also takes more effort on your part, if you are going to polish more than one car get the flex, if time is important to you get the flex.